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1 chapter 2 SOLAR PHYSICS AND TERRESTRIAL EFFECTS 7 Space Environment CenterChapter 2 The Structure of the SunAstrophysicists classify the Sun as a star of average size, temperature, and brightness a typical dwarf star just pastmiddle age. It has a power output of about 1026 watts and is expected to continue producing energy at that rate foranother 5 billion years. The Sun is said to have a diameter of million kilometers, about 109 times the diameter ofEarth, but this is a slightly misleading statement because the Sun has no true surface.

2 There is nothing hard, ordefinite, about the solar disk that we see; in fact, the matter that makes up the apparent surface is so rarified that wewould consider it to be a vacuum here on Earth. It is more accurate to think of the Sun s boundary as extending far outinto the solar system, well beyond Earth. In studying the Structure of the Sun, solar physicists divide it into fourdomains: the interior, the surface atmospheres, the inner corona, and the outer 1. The InteriorThe Sun s interior domain includes the core, the radiative layer, and the convective layer (Figure 2 1).

3 The core is thesource of the Sun s energy, the site of thermonuclear fusion. At a temperature of about 15,000,000 K, matter is in thestate known as a plasma: atomic nuclei (principally protons) and electrons moving at very high speeds. Under theseconditions two protons can collide, overcome their electrical repulsion, and become cemented together by the strongnuclear force. This process is known as nuclear fusion, and it results in the formation of heavier elements as well as therelease of energy in the form of gamma ray photons. The energy output of the Sun s core is so large that it would shineabout 1013 times brighter than the solar surface if we could see immense energy produced in the core is bound by the surrounding radiative layer.

4 This layer has an insulatingeffect that helps maintain the high temperature of the core. The gamma photons produced by fusion in the core areabsorbed and re-emitted repeatedly by nuclei in the radiative layer, with the re-emitted photons having successivelylower energies and longer wavelengths. By the time the photons leave the Sun, their wavelengths are mostly in thevisible range. The energy produced in the core can take as long as 50 million years to work its way through the radiativelayer of the Sun! If the processes in the core of the Sun suddenly stopped, the surface would continue to shine formillions of the radiative layer is the convective layer where the temperature is lower, and radiation is less is transported outward mostly by convection.

5 Hot regions at the bottom of this layer become buoyant and the same time, cooler material from above descends, and giant convective cells are formed. This convection iswidespread throughout the Sun, except in the core and radiative layer where the temperature is too high. The tops ofconvective cells can be seen on the photosphere as granules. Convective circulation of plasma (charged particles)generates large magnetic fields that play an important role in producing sunspots and 2. Thermonuclear Fusion The nuclear fusion, now occurring in the core of the Sun, turns hydrogen nuclei into helium nuclei.

6 In fact, that is howthe elements heavier than hydrogen are made; the thermonuclear fusion at the core of stars can produce the first 26elements, up to iron. The Sun, because of its relatively small mass, will go through only the first two stages of fusion,the hydrogen-helium stage and the helium-carbon 2 SOLAR PHYSICS AND TERRESTRIAL EFFECTS 8 Space Environment CenterFigure 2 1. The Structure of the Sun SunspotsPhotosphereLower ChromosphereUpper ChromosphereCoreRadiative layerPlageProminenceMagneticFieldsCorona FilamentConvectivelayerHydrogen-helium fusion can occur in more than one way, but in any case the temperature must be in the vicinity of 15million K so that two positively charged particles will be moving fast enough to overcome their electrical repulsionwhen they collide.

7 The density must be large, and the immense solar gravity compresses the gas so that it is ten times asdense as gold at the center of the Sun. If the two particles can get close enough together, the very short-range strongnuclear force will take effect and fuse them together. The most common fusion reaction in the Sun is shown in Figure2 we compare the total mass that went into this three-step fusion reaction to the total mass at the end, we will see that asmall amount of mass has disappeared. For this reaction, percent of the mass disappears and is converted intoenergy according to E = mc2 (where E = energy, m = mass and c = the speed of light).

8 The actual energy produced fromthis reaction (for a given 4 Hydrogen atoms) can be found byE = ( )(mass of 4H )c2 .In order to produce the known energy output of the Sun, 700 million tons of hydrogen are fused into 695 million tons ofhelium each second! It may be shocking to think that the Sun is losing mass at the rate of 5 million tons per second, butits total mass is so great that this rate of loss can continue for a long time (see Problem #6 at the end of the chapter ).Scientists have dreamed of being able to harness fusion energy to produce electricity on Earth.

9 In attempting the fusionprocess we are trying to duplicate the conditions in the interior of a star. There are significant problems associated withhandling a plasma at 10 to 15 million degrees. The only container that can hold material at such high temperatures isa magnetic container. At present, fusion experiments involve the confinement of a plasma in very large toroidalChapter 2 SOLAR PHYSICS AND TERRESTRIAL EFFECTS 9 Space Environment Center 1H1H2H 3He3He electronpositronGammarayFigure 2 2. The proton-proton fusion reaction which occurs in the core of the sun at a temperature of about15,000,000 K.

10 In this reaction of the total mass disappears and is released as Two hydrogen nuclei (protons) collide andfuse. One proton turns into a neutron by theemission of a positron (which has a positivecharge). The positron immediately encoun-ters its anti-particle, the electron; the pairthen annihilates, releasing two gamma result of this proton fusion is a deuteri-um nucleus, denoted 2H. 2. A deuterium nucleus collides with a proton,and they fuse to form light helium, 3He. En-ergy is released in the form of another gam-ma ray photon. 3. Finally, two 3He nuclei collide and fuse intoa nucleus of helium, 4He.